With a blank slate I am tempted to fiddle with the layout to better suit my needs (it was 75% gutted when got it).

Cabinetry, plumbing, gas, electrical I think I can do whatever I can dream.

It seems that the only thing I do not have a solution for is an alternate shower pan. The 1963 Bambi has a potty/shower combo, and an above frame backwater tank. Either the California or Ohio layout, had the shower/potty in one of the rear corners.

Is there a vendor in the industry that has figured out how to do custom pans for a reasonable price?

We had a custom stainless steel pan made for our '56 Safari (starting here) at a local HVAC shop here in town...also a black water tank support pan for our now-sold '64 GlobeTrotter. Our wet-bath set-up in the '56 is much like you explain with the black tank on top of the floor...it turned out very nice if I do say so myself.

Have you considered checking with a local boatbuilder? Anybody who builds a boat big enough to live aboard even part-time, but small enough for non-millionaires to afford, surely builds custom shower pans, or knows a source for them.

On our 65 Tradewind we did a complete change to the foor plan and made are rear bath into a side bath and rear bed. I made the pan for the shower from Tennessee cedar and fiberglassed the pan in place. We used the 1/6" fiberglass panels that you can get at the box stores for the sides. Here are some pics of that process. The photo bucket on this reply has the complete rebuild of the Tradewind.

I struggled with this one for ages. I initially intended to fab an entire cubicle including base but downsized the plan to using a store bought tub. There are a bazillion variants out there but not many for a curved wall, in the end mine is situated about 26" from the rear window which only leaves about a couple of inches curve left in the wall. Not sure this helps much but you never know.

Aktundra
We were looking for a quick and simple shower pan, but nothing fit our need and configuration of the space with the wheel well, air vent and water lines from hot water tank running through that area. What I did was template the area and we chose the cedar you could use any type of wood. After cutting all of the wood shapes I used biscut joints and glue to assemble the pan. I was not worried about any cracking with the amount of support under the pan. I used Polyester resin and 3 oz cloth to cover the pan and ran it up past the top of the pan a few inch's. This leaves a clear finish. I got the 3 oz cloth from a hobby shop it is thin enough to go around corners and not lift. I also fly RC planes and plane builders use this weight cloth and lighter to cover the model aircraft.`
You can use the Polyester resin or the epoxy, I use both depending on the task. I use the West systems Epoxy. The Polyester is the less expensive. We are in process of rebuilding a 1959 Glasspar Seafair cuddy cabin and have replaced all of the wood from the stringers,deck, bulkheads, header and seat bottoms. I have used 5 gal of Polyeseter resin on the boat which cost $100.00. West system Epoxy is $100.00 a Gal plus the hardner. There are always pros and cons for both just use the one that you feel confident with. Good luck and have fun if you take this task on.
The photo bucket on this page shows more on the pan. Dave

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